Corrosion resistant chromide coating



United States Patent 3,232,853 CORROSION RESISTANT CHROMIDE COATING Newell C. Cook, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York No Drawing. Filed Apr. 26, 1965, Ser. No. 451,053 Claims. (Cl. 204--39) This application is a continuation-in-part of my copending application Serial No. 177,196, filed March 5, 1962, now abandoned, and assigned to the same assignee as the present invention.

This invention relates to the formation of a corrosion resistant coating on a metal composition, and more particularly to a corrosion resistant coating for a metal composition wherein the coating is an alloy whose constituents comprise the metal and chromium, hereinafter called a chromide coating. Still more particularly, this invention is concerned with the formation of a chromide coating on a metal composition wherein the chromide coating comprises an alloy of chromium and the metal, and to the novel compositions obtained thereby.

Chromide coatings on a metal have been prepared by several methods. The oldest method involves packing the part to be impregnated with chromium in a mixture of powdered chromium and alumina and heating to 1300 to 1400 C. in an atmosphere of dry hydrogen for a period of 3 to 4 hours. This method is best suited for chromizing low carbon steels containing 0.10 to 0.20% carbon. The depth of a chromide coating is dependent on the temperature and time of heating.

Another method for chromizing iron, but limited to low carbon material having 0.1% carbon or less uses a mixture of hydrogen and hydrogen chloride which is passed over chromium or ferro-chromium heated to 900 C. or above to produce chromous chloride which reacts with the iron to form iron chloride and deposit the chromium. At 900 C. the rate is slow and the chromide coating is very thin, being only 0.5 mil thick after 3 hours. Thicker coatings can be obtained by using higher temperatures.

Chromizing has also been done using a fused salt bath of barium chloride and sodium chloride, or the corresponding fluorides may be used. Chromous chloride is dissolved in this bath in concentrations from 2 to 50% by weight and preferably 10 to are used. In addition, chromium powder must be present in ,the solid phase in the salt bath; otherwise chromium is not deposited. Like the preceding methods, the reaction involved is a displacement reaction in which the iron of the part reacts with the chromous chloride to deposit chromium and produce ferrous chloride, the ferrous chloride then reacting with the metallic chromium added to the bath to form additional chromous chloride and iron. Although this method can be used at temperatures as low as 900 C., the rate of chromizing is extremely slow and temperatures are preferably 1100 to 1200" C. This method is applicable to chromizing of metals other than iron.

These methods are completely unsatisfactory for the production of high precision machine parts which must be accurately machined to very close tolerances before they are chromized. The parts warp, causing the dimensions to exceed tolerance limits because of the distortion caused by the high temperatures and the phase transitions to which the articles are subjected in heating and cooling, especially when repeated steps are required to produce the desired thickness of coating. Furthermore, since these are displacement reactions in which some of the iron or other metal being chromized dissolves in order to deposit chromium to form the chromide layer, this etching of the part results in dimensional changes which are also undesirable for high precision parts.

Although it has previously been proposed to chromize iron parts by electrolyzing sodium or potassium chromium fluoride in the presence of sodium silicate using either a soluble chromium compound or alloy, or even carbon, followed by a fusion process to form the ferro-chromium alloy, this process has never found commercial utility. The reason for this, as will be shown later, is that the etficiency of actual formation of chromide layer is extremely low, with most of the chromium being deposited as non-adherent, dendritic crystals of chromium with very little of the actual chromium reacting with the base metal to form the chromide layer.

Unexpectedly, I have discovered that a uniform, adherent, tough, corrosion resistant chromide coating can be formed on a specific group of metals without an overlying layer of chromium by immersing the selected metal and chromium in a fused bath composed essentially of at least one metal fluoride and from 0.1 to 50 mole percent of at least one metal chromous fluoride (alternatively sometimes called metal fluochrornate), where the metal of both the metal fluoride and metal chromous fluoride is selected from the group consisting of alkali metals and alkaline earth metals, so that at least a portion of the bath isolates the metal from the chromium. The metal chromous fluoride may be added as such or formed in situ as explained later. I have found that such a combination is an electric cell in which an electric current is generated when an electrical connection, which is external to the fused bath, is made between the metal and chromium. Under such conditions, the chromium dissolves in the fused bath and chromium ions are discharged at the sur face of the metal where they form a deposit of chromium which immediately diffuses into and reacts with the metal to form a chromide coating. I have discovered that the rate of dissolution and deposition of the chromium is self regulating so that the chromium is never deposited at a rate faster than it diffuses and alloys with the metal. If a slower rate is desired, it can be easily controlled by means well known in the art such as by the amount of resistance in the circuit, surface area exposed to the bath, etc. A limited amount of voltage may be impressed upon the electrical circuit to supply additional direct current if a faster rate is desired.

This invention will be easily understood by those skilled in the art from the following detailed description. The metals which may be chromized by my process are those having atomic numbers 26-29 inclusive, 42, 44-47 inclusive, and 74-79 inclusive. This range of atomic numbers includes those metals included in the Periodic Chart of the Elements shown on pages: 56 and 57 of Langes Handbook of Chemistry, 9th Edition, Handbook Publishers, Inc., Sandusky, Ohio, 1956, as the Group IB metals which are copper, silver, and gold, Group VIB metals, other than chromium, which are molybdenum and tungsten, the Group VIIB metal, rhenium, and the Group VIII metals which are iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Alloys of one of these metals with at least one of the others in any proportion, or alloys containing one or more of these metals as the major phase, -i.e., over 50 mole percent, but usually over 75 mole percent and preferably at least mole percent, alloyed with any one or more of the other metals of the periodic system of elements as the minor phase constituent, |i.'e., less than 50 mole percent, but usually less than 25 mole percent and preferably less than 10 mole percent, can also be chromized 'by my process, providing that the melting point of the resulting alloy is not lower than 700 C. The fact that other metals may be the minor constituents of an alloy with the metals with which this invention is concerned does not prevent therformation of the desired chromide coating on the object. These minor constituents may be any of the other metals of the periodic system, i.e., the metals of Groups IA, 11A, IIB, IIIA, IIIB, IVA, IVB, VB, VA, and VIA, and the metals of Group VIIB not named above. These metals have atomic numbers 3-4 inclusive, 11-13 inclusive, 1923 inclusive, 25, 3032 inclusive, 37-41 inclusive, 43, 4851 inclusive, 5573 inclusive, 80-84 inclusive, and 87-98 inclusive. In the specification and claims I use the term chromide to designate any solid solution or alloy of chromium and metal regardless of whether the metal does or does not form an intermetallic compound with chromium in definite stoichiometric proportions which can be represented by a chemical formula.

The metal fluorides and metal chromous fluorides which can be used for making the fused bath are the fluorides and chromous fluorides of lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium and barium. Since it is desirable to use as low a temperature as practicable to avoid damaging or distorting the article to be chromizcd, mixtures cf one or more of the fluorides with one or more of the chrom'ous fluorides may be used to provide salt baths having lower fusion temperatures than the individual components.

In general, I prefer to have at least one alkali metal fluoride as a component of the fused salt bath. This is because the alkali metal fluorides more readily react with chromous fluoride to form the metal chromous fluoride than the alkaline earth metal fluorides. It is believed that the solubility of the chromous fluoride in my fused salt bath is dependent on the formation of the metal chromous fluoride either as a complex salt or as a double fluoride. However, the alkaline earth meta-l fluorides, except for beryllium fluoride, have higher boiling points than the alkali metal fluorides. Therefore, when it is desired to use higher temperatures, for reasons explained later, the use of the higher boiling alkaline earth metal fluorides alone, as mixtures or in conjunction with an alkali metal fluoride is preferred.

In the chromizing of metals, which are also easily beryllided, the use of beryllium fluoride will have some tendency to introduce some beryllium into the coating along with the chromium, the amount introduced being dependent on the concentration of the beryllium fluoride in the fused salt. This property can be used to produce a combined chromide-beryllide coating. To a lesser extent this etfect is also possible when magnesium fluoride is present in the fused salt bath with metals that readily form alloys with magnesium. If the deposition of this second metal, even in very minor amounts, is not desired, then I prefer that these two fluorides not be present in the fused salt bath.

Sodium fluoride and calcium fluoride are the most readily available and lowest cost metal fluorides and can be used together to form a fused bath having low melting points, low volatilization of the metal fluoride (from the bath, excellent ability to form the metal chromous fluoride and yet be used over a very wide temperature range. Therefore, these two metal fluorides are very desirable and an ideal pair of metal fluorides to be used in my fused salt bath, especially when the operating temperature is greater than 1000 C. The ratio of sodium fluoride to calcium fluoride can be varied over a wide range to obtain the desired melting point and to decrease the vapor pressure of the components of the bath.

In order to produce a reasonably fast plating rate and to insure the diffusion of the chromium into the metal to form the chromide coating, I have found it desirable to operate my process at a temperature no lower than about 700 C. and preferably at least 900 C., though the bath has a much lower melting temperature. Although temperatures lower than 700 C. may be used, there is some likelihood that the chromium will plate on-to'the surface of the metal without diffusing into the metal.

Since the rate at which chromium will diffuse into the base metal is increased by increasing the temperature, it is desirable to use as high a temperature as possible consistent with not causing distortion of the part if it has been accurately machined prior to being chromized and the ability of the materials used to construct the equipment to withstand the temperature. In those cases where distortion of the part being chromized is not a factor, e.g., the part is not accurately machined, for example, metal strips or sheets, temperatures may be used up to the flow temperature of the composition, i.e., the temperature above which the part is no longer self-supporting and the metal and/or the chromide coating on the metal will flow of its own weight.

The metal chromous fluoride is in equilibrium with the chromous fluoride and the metal fluoride. Increasing temperatures favor the decomposition of the metal chromous fluoride. However, since chromous fluoride has a boiling point exceeding 1300 C., the vapor pressure of the chromous fluoride at temperatures generally used is not sufliciently high to volatilize it from the fused salt bath.

Because of the equilibrium reaction, it is possible to form the desired metal chromous fluoride directly in the fused salt bath by dissolving chromous fluoride in the fused bath of the metal fluoride. However, since the metal chromous fluorides and chromous fluoride are not readily available because of the fact that they are readily oxidized to the corresponding chromic salt, I prefer to add the chromium salt as either chromic fluoride which readily dissolves in the metal fluoride to form the desired metal chromic fluoride, or to add potassium chromic fluoride which is readily available. This salt is then reduced to the chromous salt, either by allowing the molten fused salt to come to equilibrium with metallic chromium or more readily, by electrolyzing the bath using a scrap cathode and a chromium anode and passing the required amount of current through the molten bath until the chromic ions are reduced to the chromous state. The amount of metal chromous fluoride in the bath should normally be at least 0.1% and can be as high as 50% based on molar concentration. Usually, I use a concentration in the range of 0.25% on a molar basis. Amounts less than 0.1 mole percent can be used but adversely affect the efliciency and rate of chromide formation. Amounts greater than 5 mole percent ofler no disadvantage but represent an uneconomical amount since the higher concentration does not speed the formation of the chromide coating but does increase the partial pressure of chromous fluoride over the bath and decreases the fiuxing.

The chemical composition of the used salt bath appears to be critical. The starting salts should be as anhydrous and as free of all impurities as possible, or should be easily dried or purified by simply heating during the fusion step. The role of impurities has not been definitely established, but it appears that many things can interfere with the electrode reactions and make for poor chromide coatings. Oxygen interferes so the process must be carried out in an oxygen-free atmosphere, for example, in an inert gas atmosphere or in a vacuum. Other metal compounds can also cause the formation of poor quality chromide coatings. Best results are obtained by starting with reagent grade salts and by carrying out the process under vacuum. I have sometimes found that even commercially available reagent grade salts must be purified further in order to operate satisfactorily in my process. This can easily be done by utilizing scrap articles, preferably of the same metal to be used later, to carry out initial chromizing runs, with or without an additional applied voltage, thereby plating out and removing from the bath those impurities Which interfere with the formation of a high quality chromide coating. This may be done simultaneously with the reduction of the chromium to the chromous state. Carrying out the process in a vacuum also aids the process by volatilizing impurities and interfering substances, such as water. It is also desirable to thoroughly clean the metal surface before introduction into the fused salt, such as by pickling with or without an abrading treatment.

Although not necessary, I may use a porous, conducting container which is inert under the process conditions, for example, a graphite or Monel basket with holes, to contain the chromium as small pieces rather than to use a single, solid piece of chromium. To insure a uniform chromide coat, the chromium electrode should be spaced at least 0.25 inch, but preferably 1-2 inches from the article being chromized. In chromizing extremely large articles, for example, a sheet, in which one side may be shielded from a single chromium electrode, it may be desirable to use two or more chromium electrodes, which are judiciously spaced around the article to produce a uniform coating.

When an electrical circuit is formed external to the fused salt bath by joining the chromium to the metal to be chromized with an electrical conductor, electric current will flow through the circuit without any applied Apparently, the chromium acts as an anode by dissolving in the fused bath to produce electrons and chromous ions. The electrons flow through the external circuit formed by the conductor and the chromous ions, probably as chromous fluoride ions, migrate through the fused salt bath to the metal to be chromized where the electrons discharge the chromous ions as chromium. Because of the combined effect of the temperature of the bath and the fluxing action of the fused salts I use, the chromium immediately diffuses into the metal and forms a chromium alloy as a very smooth, adherent, tough, corrosion resistant chromide coating. The amount of current can be measured with an ammeter which enables one to readily calculate the amount of chromium being deposited on the article and converted to the chromide layer. Knowing the area of the article being plated, it is possible to calculate the thickness of the chromide coating deposited thereby permitting accurate control of the process to obtain any desired thickness of the chromide layer.

Although my process operates very satisfactorily without the impressing of an additional on the electrical circuit, I have found that it is possible to apply a small when it is desired to increase the deposition rate of chromium without exceeding the diffusion rate of chromium into the article to form the chromide layer. The impressed usually does not exceed 0.30 volt, and usually falls between 0.01 and 0.10 volt. Potentials higher than 0.3 volt frequently indicate one or more of the following conditions: high resistance somewhere in the circuit, impurities in the bath or on the surface of the electrodes which interfere with the chemical reactions at the electrodes, too fast deposition rates, loose or corroded electrical connections, etc. Although my process will operate satisfactorily when such conditions exist, it is desirable that they be corrected for more efficient operation.

, Total current densities, employing the voltages described above, should not exceed certain limits if high efficiency and high quality coatings are to be obtained. In general, the current density should not exceed ten amperes per square decimeter of cathode area. Since the diffusion rate of chromium into the cathode article varies from one material to another, with temperature and With the thickness of the coating being formed, there is also a variation in the upper limits of the current densities that may be employed. In general, however, I have found that at 800 C. current densities should not exceed approximately 0.05 ampere per square decimeter for metals in which diffusion is slow, approximately 0.1 ampere per square decimeter for metals in which diffusion is moderate, and approximately 0.25 ampere per square decimeter for metals in which diffusion is rapid. As the temperature is increased to 1000 0., current densities can be correspondingly increased to values approximately 4 to 6 times those used at 800 C. At 1150-1200 C., for those metals in which diffusion is fast, current densities up to 10 amperes per square decimeter can be used. Current densities in excess of these ranges lead to some formation of elemental chromium in either the form of nonadherent deposits or as granular or large crystalline deposits which give a rough, undesirable coating which tends to spall on further electrolysis or cooling to room temperature. Such results are desirable for the electrowinning of chromium from its compounds but are completely unsatisfactory for the production of smooth, adherent, chromide coatings on metals.

If an applied is used, the source, e.g., a battery or other source of direct current, should be connected in series with the external circuit so that the negative terminal is connected to the external circuit terminating at the metal being chromized and the positive terminal is connected to the external circuit terminating at the chromium electrode. In this way the voltages of both sources are algebraically additive.

As will be readily apparent to those skilled in the art, measuring instruments such as voltmeters, ammeters, resistances, timers, etc., may be included in the external circuit to aid in control of the process.

The following examples are given by way of illustration and not by way of limitation. It is readily apparent that variations from the specific reaction conditions and reactants given may be readily used without departing from the scope of my invention.

Example 1 This example illustrates that chromium in the chromic state cannot be used to form a chromide coating, and also illustrates how a salt bath may be produced containing chromium in the chromous state.

A salt bath composed of grams of potassium chromic fluoride and 1600 grams of a ternary eutectic mixture of 45 mole percent potassium fluoride and 45 mole percent lithium fluoride and 10 mole percent sodium fluoride was placed in a 3 inch x 10 inch monel liner in a stainless steel cell fitted with a glass dome, equipped with two ports for electrodes and another port for a thermocouple well and vacuum connection. A chromium rod approximately 6 inches long x A inch in diameter and an iron strip 18 cm. x 2 cm. x 0.1 cm. were each fastened to separate nickel rods passing through rubber tubing to form vacuum tight connections in the electrode ports. After evacuation of the container, the salt bath was melted by heating to 670 C. The electrodes were lowered so that 2 inches of both the chromium rod and iron strip were immersed in the fused salt. It was noticed that a current fluctuating between 0.5 and 0.8 ampere was generated at a potential of 0.03 volt. When the external electrical circuit was opened, an open circuit potential of 0.21 volt was established. On re-establishing this circuit, the current fluctuated between 0.35 and 0.8 ampere at a voltage of 0.04. It was noted that if the cathode was shaken, the current rose to over 1 ampere, but again decreased when the cathode was allowed to come to rest. An external was then applied to raise the voltage to 0.05 and at the same time the temperature of the bath was raised to 680 C. The current now flowing was 0.35 ampere. When the iron cathode was immersed to a total depth of 8 cm., the current rose to 0.5 ampere. Lowering the chromium electrode into the salt bath caused no noticeable increase in current or voltage. When the external circuit was again opened it was found that the open circuit voltage was still 0.21. When the circuit was again closed but without an external E.M.F. applied, the cell produced a current of 0.5 ampere at 0.05 volt. The total time of operation was approximately 30 minutes. At the end of this time, the electrodes were removed from the bath and after cooling to room temperature, were removed from the apparatus. After removing all of the adhering fused salt, it was found that the chromium electrode had lost approximately 0.55 gram, whereas the iron electrode had only gained 1 milligram in weight. This conclusively proved that essentially no chromizing had occurred either through operation of the fused salt bath as a self-generating battery or by applying an external E.M.F.

The test was repeated using platinum, nickel, iron, niobium, tantalum, and molybdenum as the cathode and a bath temperature of approximately 700 C. for a period of 2 hours and minutes and using an applied external to cause an average flow of current of 0.3 ampere for the first 40 minutes and from 0.35 to 0.4 ampere for the balance of the run. The niobium and tantalum samples actually lost weight, whereas the gain in weight of the other samples again was only several milligrams, but if chromizing had occurred to any extent the actual gain should have been approximately 0.6 gram.

These tests definitely indicated that essentially the only reaction occurring was the reduction of the chromic ion to the chromous ion and that no chromizing of any magnitude could occur until essentially all of the chromium was reduced from the chromic to the chromous state when the fused salt bath was operating either as a self-generating battery or with the application of an external E.M.F. The iron cathode was replaced with a nickel cathode, the salt bath temperature was 700 C., and an external was applied to cause a current of approximately 0.5 ampere to flow for a period of approximately 16 hours. Examination of the electrodes showed that the nickel cathode had a dendritic coat of chromium. The weight of this coat was only about 9% of the loss in weight of the chromium anode which showed that the main reaction was still the reduction of chromium from the chromic to chromous state. At this time, the applied was reduced so that the current was 0.15 to 0.16 ampere and the test continued for an additional 24 hours, after which the external was raised so that the current was 0.2 ampere and the test continued for an additional 13 hours. At the end of this time a heavy dendritic growth of chromium had formed on the nickel cathode and the chromium in the fused salt bath had been completely reduced to the chromous state. This bath was found to be very stable and the chromium was not oxidized to the chromic state during storage in a closed container between runs.

Example 2 Using the fused salt bath of Example 1 in which the chromium had been completely reduced to the chromous state but using an argon atmosphere rather than a vacuum over the fused salt bath, a coupon of cold rolled steel (15.3 cm. x 1.3 cm. x 0.15 cm.) was used as one electrode and immersed to a depth of 7.6 cm. into the fused salt bath heated to 900 C. The other electrode was the chromium rod as in Example 1. On open circuit, an of 0.044 volt was measured between the two electrodes. When the circuit was closed without any external being applied, the cell operated as a self-generating battery with the chromium being the anode and the cold rolled steel being the cathode, and generated a current of 0.06 ampere at a potential of 0.022 volt. The cell gradually reduced in current output until at the end of approximately '15 minutes, the current was 0.015 ampere at a potential of 0.004 volt. At this point an external was applied so that the current was restored to a value of 0.05 ampere at a potential of 0.013 volt. Over a period of approximately 2 hours and 20 minutes, it was necessary to increase the external step-wise to a value of 0.02 volt in order to maintain the current at 0.05 ampere. Over the balance an additional 2 hours and 45 minutes of operation the current and the voltage remained constant, at which point the electrodes were raised from the salt bath and allowed to cool to room temperature and then removed from the apparatus. After washing the cold rolled steel, it was found that the part of the coupon immersed in the fused salt bath had been coated with a very uniform corrosion resistant chromide coating. The coupon had increased in thickness by 1.6 mils or 0.8 mil per side but microscopic examination of a cross-section showed a diffused chromide coating of 1.8 mils per side. The coupon had gained 0.16 gram and there was no dendritic growth of chromium on the surface. Over a period of 7 months exposure of the coupon to the laboratory atmosphere the iron which had not been coated rusted whereas the chromide protected surface was unaffected.

Similar results are obtained when any of the other metals enumerated above as suitable for my process are substituted for the cold rolled steel. Under identical conditions of operation, thicker coatings are formed in the same length of time on some metals which are below iron in the electromotive series, for example, nickel forms a coating approximately 1.8 times as thick as that formed on iron, whereas platinum forms a coating approximately 2.1 times as thick as that on iron.

Example 3 Example 2 was repeated, except that a coupon of molybdenum (16.7 cm. x 1.3 cm. x 0.15 cm.) was used in place of the cold rolled steel as the cathode. It was immersed to a depth of 13 cm. in the fused salt bath heated to 900 C. It was allowed to operate as a battery for 10 minutes, after which an external of 0.025 volt was applied to produce a current of 0.05 ampere. Over a period of approximately 2 hours and 50 minutes, it was found that the external could be gradually decreased to maintain this current fiow so that at the end of the run the volage was only 0.018 volt. On removal, it was found that the molybdenum had increased in weight by 0.083 gram and the thickness of the sample had increased by 0.8 mil or 0.4 mil per side. Microscopic inspection of a cross-section showed that the diffused chromide layer approximately 1 mil thick had formed on each side. When the coupon was heated to 700-800 C., the uncoated molybdenum was oxidized whereas the chromide coating was not.

Example 4 The equipment for carrying out the chromiding for the following examples was modified from that used in the previous examples. The container for the fused salt was constructed of A.I.S.I. 4340 steel and was 6 inches in diameter by 18 inches deep. The top rim had a water cooled flange. The container was closed with a water cooled cover plate having two openings 1% inches in diameter for insertion of the electrodes. These holes were equipped with cover plates, electrically insulated from the main cover plate, each having a slot so that the holes could effectively be closed and yet permit the electrode wires to pass through. This container was placed inside of an electrically heated furnace. On top of each of the inlet holes, glass domes were placed having ports for insertion of the electrodes and for maintaining an inert gas atmosphere over the fused salt bath, to exclude air from the atmosphere over the fused salt bath. The non-oxidizing atmosphere was obtained by using forming gas composed of nitrogen and 10% hydrogen, by volume.

The fused salt bath was 7 kg. of sodium fluoride and 6.5 kg. of calcium fluoride, giving a mole ratio of 2 moles of sodium fluoride to 1 mole of calcium fluoride. This mixture has a melting point of 810 C. One hundred grams of chromic fluoride was dissolved in this fused salt bath, heated to 1000" C., and reduced to the chromous state, using an anode which was chrome-iron porous basket, 8 inches long by 1 inch diameter, filled with chromium flakes. A strip of cold rolled steel was immersed as the cathode and the impurities in the bath as well as the reduction of the chromium from the chro 9 mic to chromous state was obtained by applying an external sufiicient to produce a current of 0.3 ampere. At the end of 2 hours, the steel strip was removed. After increasing the temperature of the bath to 1095 C., a second sample of cold rolled steel, 15 cm. x 3.75 cm. x 0.075 cm., was placed in the bath. After 1 hour, during which time an external suflicient to produce a current of 0.3 ampere was applied. The sample gained 0.285 gram in weight due to the formation of a diffused coating of chromium compared to a theoretical gain in weight of 0.292 gram. After removal from the bath, the cold rolled steel sample had a shiny, smooth chromide coating which X-ray emission analysis showed contained approximately 35% chromium.

Example 5 Using the equipment of Example 4, a coupon of a low carbon mild steel (15 cm. X 2.5 cm. x 0.075 cm.) was chromized, using a fused salt bath temperature of 1130 C. Sufiicient was impressed upon the circuit that a current of 4 amperes flowed through the cell. In a period of 2 minutes, the sample increased in weight by 0.160 gram, which is theoretical for the amount of current. The coating was smooth and shiny in appearance, indicating that complete diffusion of the chromium into the steel had occurred. The cross-section of the sample showed that a 0.5 mil coat of chromided iron was on the surface. Both at the start and finish of the run, when no external was impressed on the circuit, the steel coupon showed a positive potential under a high resistive load, with respect to the basket containing the chromium flakes, indicating that the cell was generating current. The fact that the potential of the chromided article was positive with respect to the chromium at the end of the run, showed that the chromium had diffused into the sample.

Example 6 Example 5 was repeated except that the impressed on the cell was only sufficient for a current of 0.75 ampere to flow through the cell. After 92 minutes, the mild steel coupon had gained 1.111 grams of chromium compared to the theoretical of 1.116 grams, the coating again was smooth and shiny and a cross-section of the coupon showed a 3.8 mil diffused coating of chromium into the steel.

Example 7 Using the equipment of Example 4, but at a bath temperature of 1125 C., a coupon measuring cm. x 2 cm. x 0.3 cm. of a special alloy (Ni 65, Cr 10, Co 12, Mo 5, Ti 4, and Al 4) was chromized using an impressed to cause a current of 0.25 ampere to flow through the cell. During 1 /2 hours, the sample gained 0.554 gram in weight compared to the theoretical of 0.530 gram. Cross-section analysis of the coupon showed that it had a uniform 2 mil coating of diffused chromium. This coating had a Knoop hardness of 1000-1100 compared to 450 for the initial uncoated sample.

Example 8 Two samples of A.I.S.I. 403 steel (Cr 12.25, Mn 1.0, C 0.15, Si 0.5) each having 80 centimeters of total surface area, were chromized in the equipment of Example 4. The salt bath temperature was 1100 C. and an was impressed on the circuit to give a current of 0.3 ampere. In a period of 1% hours the samples gained 0.554 gram compared to the theoretical of 0.550 gram. The coating in each case was very smooth and metallographic inspection showed the coatings to be approximately 2 mils thick.

Example 9 A smoothly ground sample of cast iron (10 cm. x 1.9 cm. x 0.5 cm.) was chromized in the equipment of Example 4, using a salt bath temperature of 1100 C. An E.M.F. was applied to the circuit to cause a current flow 10 of 0.75 ampere. In a 1 hour period, the sample gained 1.284 gram compared to the theoretical of 0.720 gram. Metallographic analysis showed that the chromide coat was 3 mils thick.

The higher than theoretical gain in weight is believed to be due to the fact that cast iron, because of its nonhomogeneous character, provides corrosion cells in the surface of the cast iron between the iron and light elements; for example, carbon and silicon, which in themselves go into solution displacing chromium. This is based on the fact that when a sample of this cast iron was placed in the cell, with no circuit established between the cast iron and the chromium chips, a non-uniform coat of chromium was formed.

One edge of the chromide coated sample was bevelled to expose the cast iron. After exposure to the laboratory atmosphere for two weeks, the exposed cast iron surface was rusted, whereas the entire chromide coating was still bright and had not rusted or tarnished. The chromide coating on the bevelled surface was highlighted because of the rusting of the cast iron.

Example 10 A molybdenum coupon (15 cm. x 2.5 cm. x 0.05 cm.) was chromized using the equipment of Example 4, and a fused salt bath temperature of 1100 C. An was impressed upon the electrical circuit to cause a current of 0.4 ampere to flow. In 66 minutes, the sample gained 0.327 gram compared to the theoretical of 0.384 gram. The coating was very smooth and shiny. Metallographic analysis showed a 1.5 mil thick chromide coating on the molybdenum.

Example 11 Two samples of low carbon mild steel, one which had a 1 mil chromide coating formed by my process in 15 minutes using a fused salt bath temperature of 1120 C. and the other with a 2 mil coating formed by my process in 25 minutes using a fused salt bath temperature of 1120 C. were partially immersed in a stagnant 3% sodi um chloride solution at F. for 72 hours. Neither sample showed any sign of corrosion at the end of the test, whereas a sample of the steel which had not been chromized was severely corroded and rusted by such treatment.

The above examples have illustrated many embodiments of my invention. However, it will be readily apparent to those skilled in the art that other modifications can be made without departing from the scope of the present invention. For example, the chromide coating can be formed on a metal which is itself a coating on the surface of another metal; for example, an electroplate on a metal base, e.g., nickel on iron.

Because the tough, adherent, corrosion-resistant properties of the chromide coating are uniform over the entire treated area, the chromide coated metal compositions prepared by my process have a wide variety of uses. They can be used to fabricate reaction vessels for chemical reactions, to fabricate turbine blades for both gas and steam-driven turbines to resist the corrosive and erosive effects of the gaseous driving fluid, to make gears, bearings, or other articles requiring hard, wear-resistant surfaces, to protect metals, e.g., molybdenum, tungsten, etc., from being oxidized when heated to elevated temperatures, or to produce a stainless surface on metals corrodible at ambient conditions, for example, to produce a stainless steel surface on various steels, etc. Other uses will be readily apparent to those skilled in the art, as well as other modifications and variations of the present invention, in light of the above teachings. It is therefore to be understood that changes may be made in the par ticular embodiments of the invention described which are Within the full intended scope of the invention as defined by the appended claims.

What I claim as new and desired to secure by Letters Patent of the United States is:

1. A method of forming a chromide coating on a metal composition having a metal melting point of at least 700 C., at least 50 mole percent of said metal composition being at least one of the metals selected from the group of metals whose atomic numbers are 2629, 42, 44-47, and 74-79, said method comprising (1) forming an electric cell containing said metal composition as the cathode joined through an external electrical circuit to a chromium anode and a fused salt electrolyte composed essentially of at least one metal fluoride and from 0.1 to 50 mole percent of at least one metal chromous fluoride, the metal of the metal fluoride and metal chromous fluoride being at least one metal selected from the group consisting of alkali metals and alkaline earth metals, said electrolyte being maintained at a temperature of at least 700 C., but below the flow temperature of said metal composition, in the substantial absence of oxygen, (2) controlling the current flowing in said electric cell so that the current density of the cathode does not exceed substantially ten amperes per square decimeter during the formation of the chromide coating, and (3) interrupting the flow of electrical current after the desired thickness of the chromide coating is formed on the metal object.

2. The chromide coated product obtained by the method of claim 1.

3. The process of claim 1 wherein the absence of oxygen is obtained by use of an inert gas.

4. The process of claim 1 wherein the total electrical energy is self-generated within the electric cell.

5. The process of claim 1 wherein a portion of the direct current is supplied by an external impressed upon the electrical circuit.

6. A method of forming a chromide coating on a metal composition having a melting point of at least 700 C., at least 90 mole percent of said metal composition being at least one of the metals selected from the group of metals whose atomic numbers are 2629, 42, 4447, and 7479, said method comprising (1) forming an electric cell containing said metal composition as the cathode joined through an external electrical circuit to a chromium anode and a fused salt electrolyte composed essentially of at least one metal fluoride and from 0.1 to 50 mole percent of at least one metal chromous fluoride, the metal of the metal fluoride and the metal chromous fluoride being selected from the group consisting of alkali metals and alkaline earth metals, said electrolyte being maintained at a temperature of at least 700 C., but below the flow temperature of said metal composition, in the substantial absence of oxygen, (2) controlling the current flowing in said electric cell so that the current density of the cathode does not exceed substantially ten amperes per square decimeter during the formation of the chromide coating, (3) interrupting the flow of electrical current after the desired thickness of chromide coating is formed on the metal composition, and (4) removing the metal composition with its integrant chromide coating from the fused salt electrolyte.

7. The method of claim 6 wherein the metal composition is at least 90 mole percent iron.

8. The method of claim '7 wherein the metal composition is steel.

9. The process of claim 6 wherein the metal composition is nickel.

10. The process of claim 6 wherein the metal composition is platinum.

11. The process of claim 6 wherein the metal composition of molybdenum.

12. The process of claim 6 wherein the fused salt electrolyte is composed essentially of a mixture of (a) sodium fluoride, (b) calcium fluoride and (c) the metal chromous fluoride obtained by dissolving chromic fluoride in the fused mixture of (a) and (b) and thereafter reducing the chromium from the chromic to chromous state.

13. A method of forming a chromide coating on a ferrous metal composition containing at least mole percent iron which comprises (1) forming an electric cell containing said ferrous metal composition as the cathode, joined through an external electrical circuit to a chromium anode, and a fused salt electrolyte composed essentially of a mixture of (a) sodium fluoride, (b) calcium fluoride, and from 0.2 to 5 mole percent of the metal chromous fluoride obtained by dissolving chromic fluoride in the fused mixture of (a) and (b) and thereafter, reducing the chromium from the chromic to chromous state, said electrolyte being maintained at a temperature of at least 700 C. up to the flow temperature of the ferrous metal composition, in the substantial absence of oxygen, (2) controlling the current flowing in said elec tric cell so that the current density of the cathode does not exceed substantially ten amperes per square decimeter during the formation of the chromide coating, (3) interrupting the flow of electrical current after the desired thickness of chromide coating is formed on the ferrous metal composition, and (4) removing the ferrous metal composition with its integrant chromide coating from the fused salt electrolyte.

14. The method of claim 13 wherein the ferrous metal composition is at least mole percent iron.

15. The method of claim 13 wherein the metal composition is steel.

Refereuces Cited by the Examiner UNITED STATES PATENTS 1,535,339 4/1925 Peacock 20439 X 1,845,978 2/ 1932 Hasenfeld 20439 1,927,773 9/1933 Chittum 20439 2,786,809 3/1957 Raynes 20439 2,828,251 3/1958 Sibert et a1. 20439 2,957,782 10/1960 Boller 20439 2,971,899 2/1961 Hanink et a1. 20439 FOREIGN PATENTS 659,927 10/1961 Great Britain.

JOHN H. MACK, Primary Examiner. 

1. A METHOD OF FORMING A CHROMIDE COATING ON A METAL COMPOSITION HAVING A METAL MELTING POINT OF AT LEAST 700* C., AT LEAST 50 MOLE PERCENT OF SAID METAL COMPOSITION BEING AT LEAST ONE OF THE METALS SELECTED FROM THE GROUP OF METALS WHOSE ATOMIC NUMBERS ARE 26-29, 42, 44-47, AND 74-79, SAID METHOD COMPRISING (1) FORMING AN ELECTRIC CELL CONTAINING SAID METAL COMPOSITION AS THE CATHODE JOINED THROUGH AN EXTERNAL ELECTRICAL CIRCUIT TO A CHROMIUM ANODE AND A FUSED SALT ELECTROLYTE COMPOSED ESSENTIALLY OF AT LEAST ONE METAL FLORIDE AND FROM 0.1 TO 50 MOLE PERCENT OF AT LEAST ONE METAL CHROMOUS FLUORIDE, THE METAL OF THE METAL FLUORIDE AND METAL CHROMOUS FLUORIDE BEING AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF ALKALI METALS AND ALKALINE EARTH METALS, SAID ELECTROLYATE BEING MAINTAINED AT A TEMPERATURE OF AT LEAST 700* C., BUT BELOW THE FLOW TEMPERATURE OF SAID METAL COMPOSITION, IN THE SUBSTANTIAL ABSENCE OF OXYGEN, (2) CONTROLLING THE CURRENT FLOWING IN SAID ELECTRIC CELL SO THAT THE CURRENT DENSITY OF THE CATHODE DOES NOT EXCESS SUBSTANTIALLY TEN AMPERES PER SQUARE DECIMETER DURING THE FORMATION OF THE CHROMIDE COATING, AND (3) INTERRUPTING THE FLOW OF ELECTRICAL CURRENT AFTER THE DESIRED THICKNESS OF THE CHROMIDE COATING IS FORMED ON THE METAL OBJECT. 